1. Field of the Invention
The present invention relates to a video signal processing device, and more particularly, to an analog-to-digital converter for converting an analog video signal to a digital video signal having a high resolution.
2. Description of Related Art
Generally, in television and in video recording, picture information is processed using analog signal, and also is recorded and reproduced in the form of an analog signal. But, in order to carry out a high degree of picture information processing for improvement of picture quality such as noise suppression and interpolation using interline or interframe correlation, and in order to realize special effects of picture such as scale reduction and movement, a digital signal is preferred. Therefore, a video signal has been often converted to a digital signal and treated in a digital signal processing manner in broadcasting installations and industrial equipments.
This inclination also appears and is now increasing in private products. Particularly, since integrated circuit devices such an analog-to-digital converter (called "A/D converter" hereinafter), a digital-to-analog converter (called "D/A converter" hereinafter), a large capacity memory, and a high speed processor have greatly advanced and become inexpensive, the digital processing of picture information attracts a good deal of attention, and some apparatus for private use, using a video information digital processing, have already become available.
Briefly, a conventional apparatus for processing a video signal in a digital manner comprises an A/D converter receiving an analog video signal for generating a digital video signal, a digital processor receiving the digital video signal from the A/D converter for performing a required processing to the digital video signal, and a D/A converter receiving a modified digital video signal from the digital processor for generating a modified analog video signal.
In the above mentioned video information digital processing device, the A/D converter is required to convert an input video signal at a conversion rate of 20,000,000 samples per second or more in view of the nature of picture itself. Therefore, a so-called flash type of A/D converter is generally used. Assuming that the A/D converter is adapted to a parallel digital signal of n bits (n is an integer larger than 1), the A/D converter ordinarily includes 2.sup.n two-input comparators. One inputs of all the comparators are commonly connected to receive an input analog video signal, and the other inputs of the comparator are connected to 2.sup.n different reference voltages, respectively. Outputs of all the comparators are inputted to an encoder, which generates a digital signal of n bits on the basis of the results of the parallel comparison between the voltage of the input video signal and the 2.sup.n reference voltages.
As seen from the above, the high speed A/D converter which can output a digital signal of n bits needs 2.sup.n comparators and 2.sup.n different reference voltage sources. In other words, in order to obtain a required resolution, comparators and reference voltage sources of the large number corresponding to the resolution are needed. This means that the higher the required resolution becomes, the larger the required chip area for the A/D converter becomes. For example, to elevate the resolution one bit, the chip area required for the A/D converter must be doubled.
On the other hand, it is a matter of course that if the resolution is decreased, the quality of the picture will be inevitably deteriorated. Even in the case of private products, seven or eight bits of resolution are required, except for a very small picture size. In particular, a high resolution is required in a picture in which a luminance gradually and smoothly changes. In this case, if the resolution of the digital signal is low, the luminance will remarkably change at a point where the value of the digital video signal changes. This is wellknown as "false contour", and will give a nonnatural feeling to a viewer.
Furthermore, if the resolution of the digital signal is increased in a video information digital processing device, not only the A/D converter but also the digital signal processor will be enlarged in size. However, the enlargement of the digital signal processor caused by the increase of resolution is not so large, and is rather very small as compared with the increase of the A/D converter. For example, if the resolution is increased from seven bits to eight bits, the required A/D converter will be doubled in size, but the signal processor will be increased at the degree of 8/7 to (8/7).sup.2. Specifically, a circuit for multiplication is increased by (8/7).sup.2.
As can be understood from the above explanation, in order to elevate the resolution in the conventional video information digital processing apparatus, the A/D converter has to be greatly increased in size.